Project summary Human cytomegalovirus (HCMV) is a major human pathogen infecting 60-90% of the population. Its ability to establish latency and persist for the life of the host contributes to its widespread success as a pathogen. While infection in healthy adults is relatively asymptomatic HCMV can cause severe disease in immunocompromised patients such as transplant and HIV/AIDS patients and those with undeveloped immune systems. Importantly disease can be caused by virus reactivating after decades of latency when immune defenses are compromised. The viral protein UL138 is required to establish and maintain latency and is retained in all clinical isolates but lost upon serial passage in fibroblasts (25, 26, 28). UL138?s critical role during latency makes it an ideal target to combat latent infection. When expressed from a latency defective HCMV strain AD169, UL138 prevents histone deacetylase (HDAC) inhibitor valproic acid (VPA) induced expression of the viral protein IE1 from the major immediate early promoter (MIEP); blocking the first step of a lytic infection (27). This critical checkpoint determines whether HCMV will undergo a lytic or latent infection. Initial studies have found UL138 prevents lysine demethylases (KDMs) from removing repressive histone modifications on the MIEP (27). Intriguingly, UL138 localizes to the Golgi apparatus and has not been found in the nucleus where its effect on the MIEP takes place. The primary goal of this study is to identify how UL138 prevents demethylases from being recruited to the MIEP allowing for the establishment of latency. A clearer understanding of how UL138 helps HCMV establish and maintain latency is needed to develop treatments to combat latent HCMV and to clear the latent reservoir. Through our studies possible cellular targets to modulate HCMV latency will be discovered. This research is driven by the hypothesis that UL138 alters cellular trafficking of an unidentified protein(s) preventing them from recruiting KDMs to the MIEP. Supporting this hypothesis UL138 contains four Golgi sorting motifs and alters the subcellular localization of several proteins (33, 34, 35, 37). The requirement for these sorting motifs and cellular sorting machinery for UL138 mediated suppression of the MIEP will be tested and the proteins UL138 must interact with to silence the MIEP will be identified.